The present invention relates to networking technology. More particularly, the present invention relates to providing redundancy in a network for forwarding data between different hosts within such network.
Local area networks (LANs) are commonly connected with one another through one or more routers so that a host (a PC or other arbitrary LAN entity) on one LAN can communicate with other hosts on different LANs. Typically, the host is able to communicate directly only with the entities on its local LAN segment. When it receives a request to send a data packet to an address that it does not recognize as being local, it communicates through a router (or other layer-3 device) which determines how to direct the packet between the host and the destination address. Unfortunately, a router may, for a variety of reasons, become inoperative (e.g., a power failure, rebooting, scheduled maintenance, etc.). Such potential router failure has led to the development and use of redundant systems, systems having more than one router to provide a back up in the event of primary router failure. When a router fails, the host communicating through the inoperative router may still remain connected to other LANs if it can send packets to another router connected to its LAN.
Various protocols have been devised to allow a host to choose a router from among a group of routers in a network. Two of these, Routing Information Protocol (or RIP) and ICMP Router Discovery Protocol (IRDP) are examples of protocols that involve dynamic participation by the host. However, because both RIP and IRDP require that the host be dynamically involved in the router selection, performance may be reduced and special host modifications and management may be required.
In a widely used and somewhat simpler approach, the host recognizes only a single “default” router. In this approach, the host is configured to send data packets to the default router when it needs to send packets to addresses outside its own LAN. It does not keep track of available routers or make decisions to switch to different routers. This requires very little effort on the host's part, but has a serious danger. If the default router fails, the host can not send packets outside of its LAN. This will be true even though there may be a redundant router able to take over because the host does not know about the backup. Unfortunately, such systems have been used in mission critical applications such as stock trading. The shortcomings of these early systems led to the development and implementation of a hot standby router protocol (HSRP) by Cisco Systems, Inc. of San Jose, Calif. A more detailed discussion of the earlier systems and of an HSRP type of system can be found in U.S. Pat. No. 5,473,599 (referred to herein as “the '599 patent”), entitled STANDBY ROUTER PROTOCOL, issued Dec. 5, 1995 to Cisco Systems, Inc., which patent is incorporated herein by reference in its entirety for all purposes. Also, HSRP is described in detail in RFC 2281, entitled “Cisco Hot Standby Router Protocol (HSRP)”, by T. Li, B. Cole, P. Morton and D. Li, which document is incorporated herein by reference in its entirety for all purposes.
HSRP forwards data packets from a host on a LAN through a virtual router. The host is configured so that the packets it sends to destinations outside of its LAN are always addressed to the virtual router. The virtual router may be any physical router elected from among a group of routers connected to the LAN. The router from the group that is currently emulating the virtual router is referred to as the “active” router. Thus, packets addressed to the virtual router are handled by the active router. A “standby” router, also from the group of routers, backs up the active router so that if the active router becomes inoperative, the standby router automatically begins emulating the virtual router. This allows the host to always direct data packets to an operational router without monitoring the routers of the network.
Although the host is only aware of a single virtual router, the routers within the HSRP group are aware of each other. All other routers in the network also see every router in the HSRP group. That is, the routers each have their own MAC and IP addresses through which they communicate with each other. The routers of a particular HSRP group communicate with each other, for example, to determine which router is to be the active router and which is to be the standby router. Managing multiple routers having unique IP and MAC addresses within an HSRP group results in an undesirable level of complexity. Additionally, managing and ensuring scalable growth with an ever increasing number of routers within each HSRP group will likely become a significant problem in the near future. In view of the above, it would be desirable to provide an alternative redundant router scheme.